Narrow Results

Two main processes concurrently refine the nervous system over the course of development: cell death and selective synaptic pruning. We simulated large spiking neural networks (100 × 100 neurons "at birth") characterized by an early developmental phase with cell death due to excessive firing rate, followed by the onset of spike timing dependent synaptic plasticity (STDP), driven by spatiotemporal patterns of stimulation. The cell death affected the inhibitory units more than the excitatory units during the early developmental phase. The network activity showed the appearance of recurrent spatiotemporal firing patterns along the STDP phase, thus suggesting the emergence of cell assemblies from the initially randomly connected networks. Some of these patterns were detected throughout the simulation despite the activity-driven network modifications while others disappeared.

The cell assembly (CA) hypothesis has been used as a conceptual framework to explain how groups of neurons form memories. CAs are defined as neuronal pools with synchronous, recurrent and sequential activity patterns. However, neuronal interactions and synaptic properties that define CAs signatures have been difficult to examine because identities and locations of assembly members are usually unknown. In order to study synaptic properties that define CAs, we used optical and electrophysiological approaches to record activity of identified neurons in mouse cortical cultures. Population analysis and graph theory techniques allowed us to find sequential patterns that represent repetitive transitions between network states. Whole cell pair recordings of neurons participating in repeated sequences demonstrated that synchrony is exhibited by groups of neurons with strong synaptic connectivity (concomitant firing) showing short-term synaptic depression (STD), whereas alternation (sequential firing) is seen in groups of neurons with weaker synaptic connections showing short-term synaptic facilitation (STF). Decreasing synaptic weights of a network promoted the generation of sequential activity patterns, whereas increasing synaptic weights restricted state transitions. Thus in simple cortical networks of real neurons, basic signatures of CAs, the properties that underlie perception and memory in Hebb's original description, are already present.

Our best conception for the representation of learned information is the cell assembly, a self-exciting configuration of nerve cells selected from the larger network of the cerebral cortex. Despite its advantages, there is a difficulty that such assemblies may not be able to operate adequately because of insufficient connections. In the first part of this chapter, an attempt is made to define this limitation on cell assemblies, using probability calculations. The difficulty is most severe for large scale integration of cortical cell assemblies, and at the stage of formation of cell assemblies, rather than in local integration, and for the routine operation of well-established assemblies. It has previously been suggested that circuits of neural activity between cortex and hippocampus, resonating at the frequency of the theta rhythm, are important in reducing these difficulties in cell assembly function. This theory is briefly summarized. The second half of the chapter links these ideas about cell assemblies to the various concepts of the psychological impairment which follows damage to the hippocampus and related structures. These concepts include place representation, context representation, working memory, cross-temporal associations, configural representation, conditional responding and declarative memory. Despite the wide range of these various concepts, they can all be regarded as instances when the integration of activity in wide areas of cerebral cortex, and/or its standardization and stabilization over time are required. The prediction is made that psychological functions vulnerable to hippocampal damage should also be vulnerable to cortical damage in many cortical fields rather than single areas.